Monitoring DOE performance using total internal reflection

ABSTRACT

Optical apparatus includes a diffractive optical element (DOE), which includes multiple optical surfaces, including at least an entrance surface and an exit surface, and a side surface, which is not parallel to the optical surfaces of the DOE. A grating is formed on at least one of the optical surfaces so as to receive radiation entering the DOE via the entrance surface and to diffract the radiation into a predefined pattern comprising multiple diffraction orders that exit the DOE via the exit surface. An optical detector is positioned in proximity to the side surface so as to receive and sense an intensity of a high order of the radiation diffracted from the grating that passes through the side surface of the DOE.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/352,624, filed Nov. 16, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/548,476, filed Nov. 20, 2014 (now U.S. Pat. No.9,528,906), which claims the benefit of U.S. Provisional PatentApplication 61/917,953, filed Dec. 19, 2013, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to diffractive optics, andspecifically to monitoring the performance of a diffractive opticalelement (DOE).

BACKGROUND

Diffractive optics are used in a wide variety of applications. In someapplications, diffractive optical elements (DOEs) are used in creating adesired projection pattern, for purposes such as opticalthree-dimensional (3D) mapping, area illumination, and LCD backlighting.DOE-based projector designs are described, for example, in U.S. PatentApplication Publication 2009/0185274, whose disclosure is incorporatedherein by reference.

The “efficiency” of a DOE is a measure of the amount of input energythat the DOE diffracts, in relation to the energy of the incoming beam.This efficiency can vary in production due to manufacturing tolerances.It can also change during the lifetime and operation of the DOE forvarious reasons. For example, humidity and other vapors can condense onthe DOE surface and lower its efficiency, or excess heat, due to amalfunction or misuse, can deform the DOE and change its efficiency.Such changes in efficiency can result in undesirable increases in theintensity of the zero diffraction order, which is not diffracted by theprojection optics and may thus continue straight through the DOE to theprojection volume.

U.S. Pat. No. 8,492,696, whose disclosure is incorporated herein byreference, describes a DOE-based projector with a built-in beam monitor,in the form of an integral optical detector. The detector signal can becontinuously or intermittently monitored by a controller in order toevaluate the DOE efficiency and inhibit operation of the projector ifthe signal is outside a certain safe range. Such embodiments are said toprevent eye safety hazards that could otherwise arise due to DOEefficiency degradation over the lifetime of the projector.

SUMMARY

Embodiments of the present invention provide improved methods anddevices for monitoring the performance of a DOE.

There is therefore provided, in accordance with an embodiment of theinvention, optical apparatus, which includes a diffractive opticalelement (DOE), including multiple optical surfaces, which include atleast an entrance surface and an exit surface, and a side surface, whichis not parallel to the optical surfaces of the DOE. A grating is formedon at least one of the optical surfaces so as to receive radiationentering the DOE via the entrance surface and to diffract the radiationinto a predefined pattern including multiple diffraction orders thatexit the DOE via the exit surface. An optical detector is positioned inproximity to the side surface so as to receive and sense an intensity ofa high order of the radiation diffracted from the grating that passesthrough the side surface of the DOE.

Typically, the side surface is perpendicular to the optical surfaces ofthe DOE.

In some embodiments, the optical detector includes a front surface thatis in contact with the side surface of the DOE.

In the disclosed embodiments, the optical surfaces of the DOE areconfigured so that the high order of the diffracted radiation reachesthe side surface after reflecting internally within the DOE.

In some embodiments, the apparatus includes a controller, which iscoupled to receive a signal from the optical detector that is indicativeof the intensity of the high order of the diffracted radiation and tomonitor a performance of the DOE responsively to the signal. Theapparatus may also include a radiation source, which is configured todirect the radiation toward the entrance surface of the DOE, wherein thecontroller is coupled to control an operation of the radiation sourceresponsively to the monitored performance.

Typically, the controller is configured to inhibit the operation of theradiation source when the signal is outside a predefined range. In adisclosed embodiment, the diffraction orders that exit the DOE via theexit surface include a zero order, and a change of the signal isindicative of an increase of an intensity of the zero order, and thecontroller is configured to inhibit the operation of the radiationsource when the change exceeds a predefined threshold.

There is also provided, in accordance with an embodiment of theinvention, an optical method, which includes transmitting radiationthrough a diffractive optical element (DOE), which includes multipleoptical surfaces, including at least an entrance surface and an exitsurface, and which includes a grating, which is formed on at least oneof the optical surfaces so as to receive the radiation entering the DOEvia the entrance surface and to diffract the radiation into a predefinedpattern including multiple diffraction orders that exit the DOE via theexit surface. An intensity is received and sensed of a high order of theradiation diffracted from the grating that passes through a side surfaceof the DOE, which is not parallel to the optical surfaces of the DOE.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of an optical projector with a beammonitor, in accordance with an embodiment of the present invention;

FIG. 2 is a schematic pictorial illustration of an optical projectorwith a beam monitor, in accordance with an embodiment of the presentinvention; and

FIG. 3 is a schematic pictorial illustration of an optical projectorwith a beam monitor, in accordance with another embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Optical projectors based on diffractive optical elements (DOEs)sometimes suffer from the “zero-order problem,” which is described inthe above-mentioned US 2009/0185274: A portion of the input beam of theprojector (the zero diffraction order) may not be diffracted by theprojection optics and may thus continue through to the projectionvolume. Changes in efficiency of a DOE, with concomitant increases inthe zero-order intensity, can compromise system performance and may havevarious other undesirable consequences.

Any DOE comprises multiple optical surfaces, including at least anentrance surface and an exit surface. The diffractive effect of the DOEis provided by a grating formed on one of these optical surfaces (whichmay be the entrance surface, the exit surface, or an internal surfacewithin the DOE), or by multiple gratings on multiple optical surfaces.Such gratings may have any suitable shape and form, depending on thediffraction pattern that the DOE is to create. The gratings receiveradiation entering the DOE via the entrance surface and diffract theradiation into a predefined pattern comprising multiple diffractionorders that exit the DOE via the exit surface.

In a typical configuration, the grating and the optical surfaces of aDOE are arranged so that the diffraction orders that exit the DOE viathe exit surface are the lower orders (including at least the zero orderand first orders of diffraction). These orders define the diffractionpattern that is projected through the exit surface by the DOE. Thegrating may be designed to suppress the higher orders, but typically atleast some high-order radiation is diffracted from the grating at a highangle, within the DOE. (A “high order” in this context means at leastthe second diffraction order, or possibly the third, fourth, or stillhigher order.) Some of this high-order radiation may reach the sidesurfaces of the DOE, i.e., surfaces that are not parallel to the opticalsurfaces and are outside the path of the intended diffraction pattern.In many cases, these high orders reach the side surface by totalinternal reflection within the DOE.

Embodiments of the present invention that are described hereinbelow takeadvantage of this “leakage” of high diffraction orders to the sidesurfaces, typically in order to monitor the performance of the DOE. Inthe disclosed embodiments, an optical detector is positioned inproximity to a side surface of the DOE so as to receive and sense theintensity of the high-order diffracted radiation passing through theside surface. This approach is advantageous in that it enables theperformance of the DOE to be monitored using only minimal additionalhardware, and possibly with only minimal impact on the size and cost ofthe overall DOE assembly.

In some embodiments, a controller receives from the optical detector asignal that is indicative of the intensity of the high orders of thediffracted radiation, and thus monitors the performance of the DOE.Based on this signal, the controller may control the operation of theradiation source that provides the input radiation to the DOE and mayinhibit the operation of the radiation source when the signal is outsidea predefined range. For example, a decrease in the signal from thedetector may be indicative of a loss of efficiency of the DOE, which maylead to an increase in the intensity of the zero order, as explainedabove. (On the other hand, in other cases, severe failure of the DOE maylead to increased scatter, giving an increased detector signal.) In suchcases, the controller will typically inhibit the operation of theradiation source when the signal changes by more than a certainthreshold, possibly by simply turning the radiation source off.

FIG. 1 is a schematic side view of an optical projector 20 with a beammonitor, in accordance with an embodiment of the present invention. Aradiation source emits a beam 24 of radiation toward a DOE 26.Typically, the radiation is coherent optical radiation in the visible,infrared or ultraviolet range (the spectral regions that are generallyreferred to as “light”). Radiation source 22 may comprise a laser diode,for example, or an array of laser diodes, such as a vertical-cavitysurface-emitting laser (VCSEL) array.

DOE 26 comprises a transparent substrate, such as glass or a suitableplastic, for example polycarbonate, with a grating 28 formed on one ofits optical surfaces. In the pictured example, grating 28 is formed onthe entrance surface of DOE 26, facing radiation source 22, andgenerates a pattern comprising multiple diffraction orders, which exitDOE 26 through an exit surface 30. Alternatively or additionally, asnoted earlier, DOE 26 may comprise one or more gratings formed on exitsurface or on one or more internal optical surfaces (not shown). Thegratings may be configured, for example, to generate multiple, adjacentinstances of a pattern of spots, as described in U.S. Pat. No.8,384,997, whose disclosure is incorporated herein by reference. Suchpatterns are useful particularly in 3D mapping (in association with animaging assembly), as described in U.S. Pat. No. 8,384,997 and in theabove-mentioned U.S. Pat. No. 8,492,696.

In the example shown in FIG. 1, the zero, first and second diffractionorders from grating 28 make up the projected pattern, while higherdiffraction orders 32 are diffracted into the substrate volume of DOE26. These higher diffraction orders may be reflected internally betweenthe optical surfaces of DOE 26 until they reach and exit through a sidesurface 34 of the DOE. For example, in a polycarbonate substrate withindex of refraction n=1.57, the minimum angle for total internalreflection is 39.5°, so that any orders diffracted from grating 28 atangles above 39.5° will be guided by internal reflection to the sidefaces of the DOE. Typically, side surface 34 is transparent andperpendicular to the optical surfaces of DOE 26. Alternatively, the highdiffraction orders that are reflected inside the DOE may exit through alight-transmitting side surface oriented at any suitable angle that isnot parallel to the optical surfaces.

An optical detector 38, such as a silicon photodiode, receives andsenses a portion of the radiation that exits DOE 26 through side surface34. In the pictured embodiment, a collection optic 36 focuses theradiation onto the detector. Alternatively, a front surface of thedetector may be fixed in contact with the side surface of the DOE, asillustrated in FIGS. 2 and 3.

A controller 40 monitors the signal output by detector 38, which isindicative of the intensity of higher diffraction orders 32. Thisintensity, in turn, is an indicator of the efficiency of grating 28.Should the grating be degraded or fail entirely, the intensity of thehigher diffraction orders will be affected, and the signal output bydetector 38 will change accordingly. In most cases, the intensity of thehigher orders will decrease as grating efficiency drops, but in somesevere failure scenarios, the radiation scattered toward detector 38will actually increase.

If the signal is outside a certain permitted range, and particularly ifthe signal changes by more than a certain threshold—either droppingbelow a predefined minimum level or exceeding a predefinedmaximum—controller 40 will inhibit the operation of radiation source 22,and may simply turn of the radiation source entirely. In this manner,controller 40 indirectly monitors the intensity of the zero diffractionorder from grating 28, which becomes stronger as the grating efficiencydegrades, and by virtue of this monitoring is able to prevent hazardsthat could otherwise occur due to DOE failure. To perform the abovefunctions, controller 40 may comprise, for example, an embeddedmicrocontroller or even a simple threshold-sensing logic device, whichmay be integrated with projector 20. Alternatively, the functions ofcontroller 40 may be performed by a microprocessor, which also performsother functions in a system in which projector 20 is integrated.

FIG. 2 is a schematic pictorial illustration of an optical projector 42with a beam monitor, in accordance with an embodiment of the presentinvention. Projector comprises a housing 44, which contains and providespower and control signals to a radiation source, such as a VCSEL array(not shown in the figure). An optical assembly 46, mounted on housing44, includes a DOE 48, which diffracts the radiation from the radiationsource into a predefined pattern. A photodiode 50 is mounted on housing,facing inward, so as to receive high-order diffraction from the sidesurface of DOE 48.

FIG. 3 is a schematic pictorial illustration of an optical projector 52with a beam monitor, in accordance with another embodiment of thepresent invention. In this case, housing 44 and photodiode 50 are bothmounted on a substrate 56, such as a printed circuit board. Photodiode50 has an extended front surface in the form of a light guide 54, whichchannels light from the side surface of DOE 48 to the photodiode.

Although the above embodiments relate to certain DOE configurations andcertain applications of such DOEs, the use of an integral opticaldetector for monitoring a diffraction order emitted through the sidesurface of a DOE may likewise be used in other configurations andapplications. Furthermore, although the disclosed embodiments relatespecifically to applications involving projection of optical patterns,particularly for three-dimensional (3D) mapping, the principles of theseembodiments may similarly be applied in other applications in whichthere is a need to monitor the diffraction performance of a DOE.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsubcombinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

The invention claimed is:
 1. Optical apparatus, comprising: an opticalelement comprising multiple surfaces, including at least an entrancesurface, an exit surface, and a side surface, which is not parallel toeither of the entrance surface and the exit surface; a radiation source,which is configured to direct radiation toward one of the surfaces ofthe optical element; an optical detector; a light guide in contact withthe side surface so as to receive the radiation that is transmittedthrough the side surface and channel the radiation from the side surfaceto the optical detector; and a controller, which is coupled to receive asignal from the optical detector and to monitor a performance of theoptical element responsively to the signal.
 2. The apparatus accordingto claim 1, wherein the side surface is perpendicular to the entranceand exit surfaces of the optical element.
 3. The apparatus according toclaim 1, wherein the surfaces of the optical element are configured sothat the radiation reaches the side surface after reflecting internallywithin the optical element.
 4. The apparatus according to claim 1,wherein the controller is coupled to control an operation of theradiation source responsively to the monitored performance.
 5. Theapparatus according to claim 4, wherein the controller is configured toinhibit the operation of the radiation source when the signal is outsidea predefined range.
 6. The apparatus according to claim 1, wherein theoptical element comprises a grating, which is formed on at least one ofthe entrance and exit surfaces and configured to generate a predefinedpattern comprising multiple diffraction orders that exit the opticalelement via the exit surface.
 7. The apparatus according to claim 6,wherein the diffraction orders that exit the optical element via theexit surface include a zero order, and wherein a change of the signal isindicative of an increase of an intensity of the zero order, and thecontroller is configured to inhibit an operation of the apparatus whenthe change exceeds a predefined threshold.
 8. The apparatus according toclaim 1, and comprising a circuit substrate, wherein both a housing ofthe radiation source and the optical detector are mounted on the circuitsubstrate.
 9. An optical method, comprising: providing an opticalelement comprising multiple surfaces, including at least an entrancesurface, an exit surface, and a side surface, which is not parallel toeither of the entrance surface and the exit surface; directing radiationtoward one of the surfaces of the optical element; channeling theradiation that is transmitted through the side surface of the opticalelement through a light guide that is in contact with the side surfaceto an optical detector; and monitoring a performance of the opticalelement responsively to a signal output by the optical detector inresponse to the radiation received through the light guide.
 10. Themethod according to claim 9, wherein the side surface is perpendicularto the entrance and exit surfaces of the optical element.
 11. The methodaccording to claim 9, wherein the surfaces of the optical element areconfigured so that the radiation reaches the side surface afterreflecting internally within the optical element.
 12. The methodaccording to claim 9, wherein the radiation is directed toward the oneof the surfaces of the optical element by a radiation source, and themethod comprises controlling an operation of the radiation sourceresponsively to the monitored performance.
 13. The method according toclaim 12, wherein controlling the operation comprises inhibiting theoperation of the radiation source when an intensity of the sensedradiation is outside a predefined range.
 14. The method according toclaim 9, wherein the optical element comprises a grating, which isformed on at least one of the entrance and exit surfaces and configuredto generate a predefined pattern comprising multiple diffraction ordersthat exit the optical element via the exit surface.
 15. The methodaccording to claim 14, wherein the diffraction orders that exit theoptical element via the exit surface include a zero order, and wherein achange of the intensity of the sensed radiation is indicative of anincrease of an intensity of the zero order, and the method comprisesinhibiting emission of the radiation from the optical element when thechange exceeds a predefined threshold.